This invention relates generally to functional materials, such as shape memory alloys, and more particularly relates to control of energy dissipation in a superelastic shape memory alloy structure.
The degree of energy dissipation, or energy damping, in a functional material, such as a shape memory alloy (SMA), has important practical implications for many SMA devices and systems. For a wide range of SMA applications a high degree of damping can be desired, e.g., for enabling impact absorption and vibration control. In contrast, in other SMA applications, including mechanical actuation and energy harvesting, energy damping can be undesirable. Aside from this consideration for energy dissipation, the design specifications for a SMA application as a whole or a SMA active element in an application system can otherwise be similar. For example, a SMA wire element design that is designed for actuating the movement of a mirror is can also operate in a woven fabric that is designed dissipating energy from the vibrations in an engine mount.
Shape memory alloys are characterized by a solid-to-solid reversible phase transformation between a higher temperature phase, called austenite, and a lower temperature phase, called martensite. The alloy crystal structure of the austenitic phase is typically a cubic superlattice, while the alloy crystal structure of the martensitic phase is monoclinic or orthorhombic. The transformation of the alloy material between these two phases results in recoverable strains on the order of 6-10%. During such a so-called martensitic transformation, energy is dissipated as heat; the amount of this energy dissipation is reflected in the degree of hysteresis in a phase transformation cycle: the larger the phase transformation cycle hysteresis the more energy is dissipated by the phase transformation cycle. As a result, the amount of energy damping produced by a shape memory alloy structure in a phase transformation cycle can be measured by the size of the hysteresis in a stress-strain curve for a phase transformation cycle that is obtained during an observed mechanical phase transformation of the structure.
For a range of SMA applications a large hysteresis can be desirable, e.g., for applications in which the function of the SMA is to damp vibrational energy. In such applications for which energy dissipation is desirable, a SMA material element can be correspondingly engineered to damp mechanical energy. But although a SMA damping design can in general be effective, it typically requires a trade-off with other SMA material properties, such as mechanical fatigue and corrosion resistance. Similarly, a SMA material element can be engineered to produce relatively low mechanical damping, but also at a trade-off with other SMA material properties, such as temperature sensitivity, mechanical stresses, cost, or manufacturability. Due to these inherent trade-offs required in the design of a SMA material element with a selected degree of damping, control of SMA material element damping is often impractical, and results in a common SMA material element design being employed for both high-damping and low-damping applications; e.g., with a substantially identical SMA element design being employed for both for actuating structures and for energy dissipating structures.
The trade-offs required for achieving a particular, prespecified degree of energy damping are particularly large for microscale and nanoscale SMA structures having SMA material element dimensions in the microscale or nanoscale. For such SMA structures, the material requirements set by operational and performance considerations can be very stringent. In particular, the compromises that are often required for achieving various small-scale operational performance can result in an inability to selectively control energy damping by the SMA structure. As a result, microscale and nanoscale SMA material structures can be severely limited in meeting specific energy damping requirements given for microscale and nanoscale SMA applications.